Method of reducing the shrinkage of wood



Patented June 23, 1936 UNITED STATES PA'IZENT OFFICE I 3,0,850 i METHODOF REDUCING THE SHBINKAGE F WOOD Ma :1. uam, mdgewood, and AlfredBurgeni. East Orange, N. 1., assignors to Wallwood Gorporatlon, NewYork, N. Y., a corporation of New York a No Drawing. Application January8, 1936,

Serial No. sam

It has been known in the fm of wood treating that relatively dry wood,even though kilndried,

will reabsorb moisture from the atmosphere and r such a way as toeliminate shrinkage, but to our knowledge none of these efforts has beensuccessful, at least to the (same extent as the method described herein.For example, when sheets-of wood veneer, treated according to previouslyI known methods, are secured to a surface ofsubstantially constant area,that is to say, one which is substantially non-shrinkable, suchveneering will shrink to such an extent as to make itmost V difficult tohold the veneering on the surface to which it is secured. The glue orother adhesive used for securing the veneering to such surface I isusually not strong enough to hold thevenecr-y ing and prevent itsbuckling or cracking; asthe case may be, depending upon whether thewood, 1 expands or contracts.

If, however,-such veneering can be so treated that itsshrink'age isnegligible, say less than 1%, then there is no trouble about the glueproperly holding the veneering in place. The treatment incidentallyresults in making the wood more flexible than untreated wood, the amountof flexibility naturally depending largely upon the dimensions of thepieces of wood which are treated.

While we have referred to a piece of veneering as being secured to asubstantially constant area,

of course what we have said above is also true I where such surface hasa shrinkage of a substantial amount and the veneer itself has ashrinkage different than that of the surface. The

" difference in shrinkage is the important thing,

and we have found that by our process this shrinkage can be keptwithirrany desired amount so as to avoid buckling orcracking. Normally,the difference in shrinkage should be less than about 1%. For example,if the shrinkage of the base material is 1%, then that of the veneeringshould not exceed 2%.

While the invention finds a wide use in conmotion with veneering,nevertheless if wood in other forms than veneering isjtreated so thatits shrinkage is negligible, it becomes valuable for many architecturaland other purposes, which need not be enumerated, since they willreadily suggest themselves.

We have found that we can obtain our novel product by subjecting wood tothe following steps.

First it is preferable to treat the wood with an alkali and then with anacid, preferably a volatile one. The wood then may be subjected todrying and ironing, after which it is impregnated in such a way that thefibers of the wood are saturated without leaving any substantial amountof impregnator in the cell cavities of the wood. Then the wood-is drieda second time and also ironed again. The foregoing method is one whichhas been found satisfactory, although in many instances certain stepscan be omitted, and this brief outline of the one method is not intendedas limiting the invention to that one method. We will now proceed todescribe the steps more in detail.

According to our invention, weilrst subject the woodto an alkalitreatment. This alkali may 'be any one of several well-known alkaliessuitable for the purpose. For example, we may use sodium hydroxide,potassium hydroxide, or ammonia, to name several which we have found tobe suitable, both as to results and as to cost.

The concentration of the alkali and the temperature and duration of thetreatment with the alkali, will naturally vary with the different kindsof. woods. They will also depend to a large extent upon the amount offlexibility which is desired for the final product. The examples whichwill be given later will serve as a guide to those skilled in the art.However, the concentration of the alkali and the temperature andduration of the treatment are so selected as to dissolve and remove asubstantial part of the resin, pectin, lignin, and other similarcomponents of the wood and so as to leave the cellulose fibers free toany desired degree from such components, which usually encrust thefibers. The above factors of concentration of alkali and temperature andduration of thetreatment have an important relation to the shrinkage ofthe product after the later predrying step, and itis therefore importantthat these factors be so selected as to keep such shrinkage from beingexcessive.

correct figures for the above factors. These figures may be determinedby graphical or other analysis of the figures obtained from thepreliminary tests. We believe it to be essential that such preliminarytests shall be continually run, to control our method.

Usually, the alkali treatment is carried on at atmospheric pressure,with a temperature below 212 F., or, in other words, below thetemperature of boiling water. Preferably also, the alkali concentrationis relatively low compared to most concentrations previously used inwood treatment. In most instances, we prefer to keep this concentrationof alkali below 3%, and in many instances it is much lower than 3%. Itshould be at a low enough concentration to avoid softening of the woodfibers to such extent as to cause them to lose their continuity asfibers. In any event, the concentration of alkali and temperature andduration of the treatment are such that the natural wood structure isnot destroyed to any substantial extent.

The alkali treatment may be carried out by any usual apparatus. Forexample, if thin pieces of wood are being treated, they may be soaked ina suitable container containing an aqueous solution of the alkali, and,if desired, the alkali treatment may also include the use of vacuum orpressure, or both, in order to obtain a more ready and uniform reactionthroughout the body of the wood.

The immediate result of the alkali treatment is to cause the fibers ofthe wood to be greatly swollen and softened, and possibly other changestake place. Among these other changes it may be noted that the wood isusually darkened in color. The alkali appears to react with the resin,pectin, lignin, etc., which for the sake of convenience we shallhereinafter refer to as the noncellulosic constituents of the wood, andat any rate to make these constituents more easily removable, leavingthe fibers free to be reached easily by the impregnating materialsubsequently used.

The next step following the alkali treatment preferably is to rinse thewood in water to remove the excess or superficial alkali. Usually, a fewminutes washing is sufiicient for this purpose. Preferably, the wood isthen immersed in an aqueous solution of a volatile acid, by which ismeant an acid having a boiling point less than 300 F. While under someconditions it may be possible to use a non-volatile acid, we have foundthat a volatile acid is highly preferable. Among such acids which aresatisfactory, we have used formic, which we find to be preferable,acetic, propionic, dilute hydrochloric, hydrobromic, and hydriodic. Withsome woods it may be possible to use a non-volatile acid. In this casethe acid must be removed by washing before the wood is dried. While thiswashing has the effect of increasing the shrinkage of the wood at thisstage of the process, it may be desirable, with some woods whose naturalshrinkage is comparatively low, to employ this method.

The object of this acid treatment is to neutralize any residual alkalithat may still be in the wood, and also to bleach the wood to itsoriginal color. The acid treatment likewise deflates the cellulosicfibers of the wood which have been swollen by the alkali treatment.

The concentration of the acid and its temperature and the duration ofthe treatment should be determined by a previous test of samples fromthe batch being treated. Samples of wood may be subjected to variousconcentrations, temperatures, and duration of treatment until thedesired neutralizing, bleaching, and deflation effects have beenobtained.

We find that sometimes the action of the acid may be improved byapplying it under pressure, which will cause it to more readilypenetrate to the interior of the wood.

We next subject the wood to a drying action which, among other things,results in evaporation of the acid. It will be seen that the evaporationis greatly expedited by using a volatile acid in the previous step, asreferred to above. By the use of such an acid, it is unnecessary tosubmit the wood to washing to remove the acid from the wood. A washingstep at this stage is very undesirable for most woods, because it wouldcause the wood fibers to swell, which we desire to avoid.

The drying may take place in any suitable apparatus, such as a dryingtower or kiln, either with or without recirculation of the air thereinand either with or without a vacuum.

Next, the wood may be passed through ironing rolls, where it issubmitted to a substantial amount of pressure. These rolls may be coldor heated, and are preferably of steel or other hard material so as toperform an ironing action on the wood, thus more completely drying thewood and setting the fibers in their new size and interrelation. This isthe result partly of a further deflation of the fibers from theirswollen condition produced in the alkali. In many instances, however,this ironing step may be omitted. The ironing may be considered as apart of the drying step and so in the claims for the sake of concisenesswe shall use the expression drying to mean either drying, or ironing, orboth, as described above.

After the above steps have been performed, we find that, for most woods,the tendency and ability of the wood to adsorb or absorb moisture andtherefore to swell and shrink is greatly retarded and is permanentlyreduced to a substantial degree. As indicated above, one of the reasonsfor the improved result obtained at this stage is the fact that we haveavoided washing with water to remove the acid. It is known that alkalisolutions swell wood more than water, but, on the other hand, 'it isalso known that such swelling can be retained when the swollen wood issoaked in water after removal of the alkali. Thus, by avoiding the useof water to remove the acid, and by thoroughly drying the wood at thisstage, the swelling of the wood is greatly reduced. Likewise, the woodappears to be made stronger and more elastic, and consequently a betterand more serviceable product results.

The wood is now ready for impregnation, and this we preferably do by theuse of one or more foreign substances, impregnating the wood to thefiber saturation point for water at any desired relative humidity of theatmosphere, or to any desired predetermined degree below the fibersaturation point. In other words, we use a sufficient quantity ofimpregnator to saturate the individual fibers of the wood without anysurplus, thus leaving the cell cavities or spaces between fiberssubstantially free of impregnating substance after the subsequent dryingstep. We found that by so doing we can obtain a wood in which theshrinkage is reduced to such a negligible figure that the wood will notshrink, crack, or blister. Certain tests have been worked out and aregenerally accepted as determining fiber saturation, and we employ thosetests for that purpose. Such tests are disclosed in Technical BulletinNo. 282, referred to hereafter.

By impregnating the fibers, they seem to be swollen to a certain amount,which is relatively small, particularly as compared to the swellingcaused by the alkali treatment, and then they resist the adsorption offurther moisture from the atmosphere. In this connection, we shall usethe term "adsorption" in preference to absorption, as the former appearsto be the action which takes place, although, of course, the name givento this action is immaterial. At any rate, the presence of theimpregnator in the fibers and its relative lack in the cells between andamong the fibers seems to very materially reduce the shrinkage in theultimate product.

The other steps of the process aid very materially in obtaining asatisfactory result, but in some instances we have been able tosubstantially eliminate the shrinkage by controlling the impregnation ofthe fibers as just outlined above, without any of the preceding stepsdescribed above.

At the present time the most suitable impregnator which we have found isa substance known commercially as glycerine, and chemically as.

glycerol", or some equivalent substance, such, for example, as glycol orother polybasic alcohol or some substituted polybasic alcohol. Suchproducts are non-volatile and are also miscible with water andhygroscopic. They are also the kind of liquids which may be classifiedas polar, and we have found'that such liquids are preferable asimpregnators. The solvent used with the impregnator is usually water,although naturally the particular solvent used with any particularsubstance would depend largely upon that substance.

We are not certain as to the correct explanation as to why this type ofimpregnator is desirable, but wedo find that it is. Possibly the correctexplanation is that a polar liquid is readily adsorbed by the cellulosicwood fibers, and will cause the wood to swell to a relatively smallamount, but sufilcient so that the liquid may thoroughly permeate thefibers. Non-polar liquids, on the other hand, are not adsorbed by thewood fibers to any substantial degree and do not cause those fibers toswell so that the impregnator may thoroughly permeate them. I

Assume that the fibers have been impregnated with a polar liquidimpregnator, for example one which is soluble in water, as most suchimpregnators are. Assume also that a sufllcientquantity of impregnatorhas been used to saturatethe fibers at the relative humidity selected asthe minimum above which shrinkage is to be substantially eliminated. Forexample, assume that/20% relative humidity has been selected as'thedesirable figure.

Now if the humidityof the atmosphere be re- I duced below 20%, watervwill evaporate from the impregnator and the wood will shrink. On theother hand, if the humidity of the atmosphere inof the atmosphere isagain ;.re duced, the water set forth above.

However, if care has been taken to impregpreviously taken up by the woodwill evaporate while the impregnator becomes more concentrated, asbefore, and is readsorbed by the fibers, and without substantialshrinkage of the wood.

Whether or not the above explanation is the correct one, we find thatexcellent'results are obtained by following the method outlined above,shrinkage at andabove the selected'relativo humidity being reduced to anegligible amount as In the claims, it will of course be understood thatthe expression selected relative humidity" refers to this minimum, abovewhich shrinkage is so reduced.

Still further improved results may be obtained by first using animpregnator which is non-water soluble, using enough of that substanceto reach the limit of saturation of the wood for that particularimpregnator. Then the wood may be still further impregnated with thewater-soluble impregnator referred to above, up to the limit ofsaturation of the wood for that substance. That is to say, betterresults are obtained by first using a non-hygroscopic impregnator, andthen a hygroscopic impregnator. The first does not cause the fibers toswell as much as the second, and therefore does not permeate the fibersto such an extent as the second.

We believe that the correct explanation as to the improved resultsobtained by the use of two impregnators is as follows: Assume that asingle hygroscopic impregnator is used to saturate the fibers at 20%relative humidity without any substantial excess of impregnator in thecells between fibers. Then assume that the humidity of the atmosphererises. The hygroscopic impregnator will absorb the moisture in the airandwill overfiow or ooze out from the fibers into the cells, and if thehumidity becomes great enough, will give the wood a feeling of wetness.If, on the other hand, part of the impregnator is non-hygroscopic, thenproportionally less water from the atmosphere can be absorbed, and theycell cavities will not be as nearly filled, and therefore there is lessliability of moisture becoming evident.)

For the hygroscopic impregnator, we find that glycerine or itsequivalent is preferable, using water as a solvent, and for thenon-hygroscopic impregnator we may use paraflln, beeswax, or the like.As a solvent for the non-hygroscopic impregnator, we use a water solublepolar solvent, such as ethylene glycol monoethyl ether.

The hygroscopic impregnator may be used by itself in combination withthe other steps of our method, and excellent results canbe obtained. butwhere the range in humidity towhich the finished product is to besubjected is very great, then we find it advisable,to use the twoimpregnators as just described.

The concentration of the impregnating solution, in any case, should becontrolled in order to produce the desired freedom from shrinkage in thefinished product. First we selectthe minimum'percentage of .relativehumidity at which the product is to be stabilized, that is, to bemaintained substantially free from shrinkage. Then it -March, 1932,entitled Strength-moisture relations for wood", by T. R. C. Wilson, ofthe Forest -Products Laboratory.

For example, we impregnate samples of the particular batch of wood to betreated, using various percentages of the impregnator and measuring theshrinkage of the various samples. From the data thus acquired, we mayplot a curve similar to those shown on pages 50 to 53 of the aforesaidbulletin, and from that curve may obtain the impregnator contentcorresponding to fiber saturation, and also the impregnator content atthe intersection point, which latter is usually only slightly greaterthan the former. The intersection point is defined on page 12 of saidbulletin as the point where the inclined and horizont 1 lines of thecurve intersect.

We next determine by experiment the concentratlon of impregnatingsolution which is required so that the impregnating solution which isretainedby the wood immersed therein will contain the amount ofimpregnator necessary to saturate the fibers, to correspond to theintersection point, or to correspond to any other shrinkage pointdesired.

Having thus determined the desired concentration and amount ofimpregnating solution, the wood is immersed in a solution of therequired concentration until that approximate amount has been adsorbedby the wood. Sometimes it is difficult to cause the wood to adsorb asuflicient amount of impregnator, particularly after thewood has beendried or ironed. In order to overcome this difliculty, the wood may befirst soaked in water before impregnating. This soaking does not restorethe wood to as swollen a condition as that in which it left the alkalisolution, because the previous drying operation has greatly retarded thetendency and ability of the wood to adsorb moisture and therefore toswell. In fact, this tendency and ability has been permanently reducedto a substantial degree. After a preliminary soaking in water, if theimpregnator to be used is water-soluble, the wood may be placed directlyin the impregnating solution. If on the other hand, the impregnator tobe used is nonwater soluble, it is preferable to place the wood first ina bath of the pure solvent used for the particular impregnator, so thatthat solvent may displace the water in the wood, and then the wood maybe placed in a bath of impregnator dissolved in such solvent.

The next step after impregnation is a second drying, which may becarried out in any suitable drying apparatus and which results in theevaporation and removal of the solvent for the impregnating material andthe retention within the wood fibers of the non-volatile impregnator.The

amount of the remaining impregnator is enough to saturate the individualfibers of the wood without any substantial surplus, as pointed outabove.

The final step of the process consists in again ironing the wood bypassing it through a series of rolls of steel or ot er hard material,which may be either heated or cold and which are so adjusted as to exerta suitable pressure on the wood. The amount of pressure in this ironingstep, as in the preceding one, is on the order of that given by acommercial ironing machine, such as used in laundries.

The ironing at this stage seems to compress the wood and to finally setthe fibers in their permanent condition and interrelationship. It alsoseems to result in a diminution of the swelling and shrinkage (which forsimplicity we are referring to herein as shrinkage) which the woodundergoes between the stabilization point of relative humidity andrelative humidity. This final ironing step also seems to make the woodstronger and more elastic than in its raw condition.

We will now give a few examples of treatments which have been performed,which we believe will furnish a sufficient guide to those skilled in the5 art so that they may practice the invention. Each example indicates apiece of veneer, although similar results would of course be obtainedwith thicker wood.

Example I I 10 A piece of white pine veneer was treated with alkali inthe form of a 0.48% sodium hydroxide solution at 177 F. for two hours,after which it was subjected to a ten minute treatment with formic acidof a 2.38% concentration. The sample was then suspended for one hour ina current of dry air at about F. It was then permitted to soak in waterover night and then impregnated with glycerine for two hours, theglycerine having a specific gravity of 1.0770. The sample was then driedby again being suspended in a current of dry air at 150 F. for one hour,after which it was ironed by being passed through an ordinary laundryironing machine, at as great a pressure as could be conveniently usedwithout tearing the wood.

The result was a piece of veneering in which the shrinkage wassubstantially zero as compared to a shrinkage of 3.5% in the rawveneering before treatment.

Example II Example III 45 A sample of white birch veneer was treated fortwo hours in a 0.125% solution of sodium hydroxide, and then for tenminutes was treated with a 3.05% solution of sulfuric acid, after whichit,was dried and soaked in water as in Example I. It was thenimpregnated with glycerine, this treatment lasting for two hours and theglycerine having a specific gravity of 1.1256. The sample was then driedand ironed as in Example I. The product showed a shrinkage of 0.8% asagainst a shrinkage in the original piece of 9.6%.

We believe that the above examples, with the detailed description of ourmethod given above, will be suificient for those skilled in the art to 0practice the invention. Numerous samples have been treated, and we findthat by properly controlling the various steps as outlined above, nodifficulty is had in obtaining products of various kinds of woods havingthe desired shrinkage. In 5 each case, the resulting sample was flexibleenough for use as veneer, and usually the flexibility is increased bythe treatment, as is also the resiliency and strength. This may or maynot be the case, however, in all instances, because 70 the principalthing sought for is a control of the shrinkage, and so flexibility,resiliency, or strength may be sacrificed in the interests of shrinkage,if found necessary. Usually, however, no material sacrifice in thoseproperties is 76 called for, and in fact usually there is an improvementof those properties by the treatment. Where a piece of wood is to bejoined to the surface of another material, the amount of shrinkagerequired would depend upon the shrinkage of that surface, as pointed outabove.

In the three samples given above, it will be seen that in each case weemployed the step of soaking the wood in water before impregnation andafter drying and that we also omitted the first ironing step. In someinstances, we find that predrying is sufiicient without the ironing, andin other instances we find that the soaking in water after drying canalso be omitted. That soaking was allowed to continue over night merelyas a matter of convenience, as in many other examples we have found thata much shorter time, for example six hours, is enough.

While we have described the invention more or less in detail, it is tobe understood that those details can be varied within wide limitswithout departing from the scope of the invention as delined by theappended claims.

We claim:

1. The method of reducing the shrinkage of wood which comprises thesuccessive steps of substantially removing the non-cellulosic substancesfrom the wood, as by an alkali treatment or the like, while preservingthe natural structure of the wood, treating with glycerol or a likenonvolatile impregnator carried by a relatively volatile vehicle, andthen drying to drive off said vehicle, the amount of said impregnatorbeing suflicient to substantially saturate the wood fibers at a selectedrelative humidity, and after said drying operation leaving the cellcavities of the wood substantially free of the impregnator.

2. The method of reducing the shrinkage or wood which comprises thesuccessive steps 01' substantially removing the non-cellulosicsubstances from the wood, as by an alkali treatment or the like, whilepreserving the natural structure of the wood, drying to set the fibers,treating with glycerol or a like non-volatile impregnator carried by arelatively volatile vehicle, and then drying to drive off said vehicle,the amount oi said impregnator being suflicient to substantiallysaturate the wood fibers at a selected relative humidity, and after saidsecond drying operation leaving the cell cavitiesof the woodsubstantially free of the impregnator.

3. The method of reducing the shrinkage of wood which comprises treatingit with glycerol or a like non-volatile impregnator carried by arelatively volatile vehicle, and then drying to driveofi said vehicle,the amount of said impregnator being sufiicient to substantiallysaturate the wood fibers at a selected relative humidity, and after saiddrying operation leaving the cell cavities of the wood substantiallyfree of the impregnator.

4. The method of reducing the shrinkage of wood which comprisessaturating the fibers with two impregnators, first with paraffin or alike non-hygroscopic one and then with glycerol or a like highlyhygroscopic one, each carried by a relatively volatile carrier, anddrying, the amount of each impregnator being sufiicient to substantiallysaturate the wood fibers at a selected relative humidity, and after saiddrying operation leaving thecell cavities of the wood substantially freeof that impregnator.

5. The method of reducing the shrinkage of wood which comprisessaturating the fibers with two impregnators, first with paraflin or alike nonhygroscopic one and then with glycerol or a like highlyhygroscopic one, each carried by a relatively volatile carrier, and thendrying, the total amount of impregnator used being sullicient tosubstantially saturate the wood fibers at a selected relative humidity,and after said drying operation leaving the cell cavities of the woodsubstantially free of impregnator.

6 The method of reducing the shrinkage of wood which comprises thesuccessive steps of substantially removing the non-cellulosic substancefrom the wood, as by an alkali treatment or the like, while preservingthe natural structure of the wood, treating with a volatile acid anddrying to set the fibers, treating with glycerol or a like nonvolatileimpregnator carried by a volatile vehicle, and then drying to drive ofisaid vehicle, the

amount of said impregnator being sufiicient to substantially saturatethe wood fibers at a selected relative humidity, and after said seconddrying operation leaving the cell cavities of the wood substantiallyfree of the impregnator.

7. The method of reducing the shrinkage of wood which comprises thesuccessive steps of substantially removing the non-cellulosic substancefrom the wood, as by an alkali treatment or the like, while preservingthe natural structure of the wood, treating with a volatile acid anddrying to set the fibers, and treating with two impregnators, first withparaifin or a like non-hygroscopic one and then with glycerol or a likehighly hygroscopic one, each carried by a relatively volatile carrier,and drying, the total amount of impregnator being sufllcient tosubstantially saturate the wood fibers at a selected relative humidity,and after said second drying operation leaving the cell cavities of thewood substantially free of impreglike, while preserving the naturalstructure of the wood, treating with glycerol or a like non-volatileimpregnator carried by a relatively volatile vehicle, and then drying todrive oil. said vehicle, the amount of said impregnator being sufficientto substantially saturate the wood fibers at a selected relativehumidity, and after said drying operation leaving the cell cavities ofthe wood substantially free of the impregnator, and finally ironing theresulting product.

9. The method of reducing the shrinkage of wood which comprises thesuccessive steps of substantially removing the non-cellulosic substancesfrom the wood, as by an alkali treatment or the like, while preservingthe natural structure of the wood, drying to set the fibers, and thenimpregnating with glycerol or a like non-volatile impregnator.

10. The method of reducing the shrinkage of wood which comprises thesuccessive steps of substantially removing the non-cellulosic substancesfrom the wood, as by an alkali treatment or the like, while preservingthe natural structure of the wood, treating with a volatile acid anddryi: to set the fibers, and then impregnating with glycerol or a likenon-volatile impregnator.

HAROLD H. GRIFFIN. ALFRED BURGENI.

